Centrifugal fan

ABSTRACT

In a centrifugal fan, an impeller portion, fitted to a rotor portion which rotates with respect to a stator portion, rotates around a central axis J 1 . The impeller portion is housed in a housing comprised of a housing body and a cover portion. By rotation of the impeller portion, air is taken in from an inlet and delivered from an outlet. In the centrifugal fan, an annular protrusion, protruding toward a connection plate on the lower side of blades, is formed in the base portion in order to make a space between the impeller portion and the base portion small. A depression for partially expanding a space between the connection plate and the base portion is provided in the annular protrusion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motorized centrifugal fan to be used for air blasting.

2. Description of Related Art

In a centrifugal fan for taking in air in an axial direction and discharging the air in a radial direction, there has hitherto been provided an impeller having a plurality of blades arranged in a circumferential direction with a rotational axis as the center, and while a cup-shaped portion provided at the midsection of the impeller has a field magnet on its inside surface, a cylindrical metal yoke having magnetism is pressed and fitted to the cup portion, to make the impeller rotatable around the rotational axis. A variety of devices are comprised in the centrifugal fan as thus described in order to reduce noise, improve an air volume characteristic, and suppress performance degradation with the increase in temperature of a motor.

A rotational speed of a recent centrifugal fan for cooling is on the increase, and since a current of a motor housed inside the centrifugal fan increases with the increase in rotational speed, temperatures of an armature and an electronic component tend to increase in too large a degree to be ignored. When requesting for further increasing the rotational speed and using the centrifugal fan under high temperature environment are considered, it is necessary to cool the armature and the electronic component by force.

BRIEF SUMMARY OF INVENTION

A centrifugal fan of the present invention comprises: a substantially bottomed cylindrical cup portion, including a cylindrical yoke portion with a prescribed central axis as the center; a field magnet, arranged inside the cup portion and fixed to the cup portion; an impeller portion, connected to the cup portion, having a plurality of blades arranged around the central axis outside the cup portion, and rotated along with the cup portion to take in air from the bottom side of the cup portion and discharge the air in a direction separating from the central axis; a housing, having a side wall portions which covers the periphery of the impeller portion and has a pair of ends in a circumferential direction, a base portion which covers the opening side of the cup portion and a cover portion in which an inlet opposed to the bottom of the cup portion is formed, the housing being where a width of a flow channel between the side wall portion and the impeller portion gradually expands toward an outlet as an opening between the pair of ends; a bearing mechanism, which rotatably supports the cup portion with the central axis as the center with respect to the base portion; and an armature, which is fixed to the base portion and at least partially located inside the cup portion, and generates a torque with the central axis as the center between the located position and the field magnet.

Further, in the centrifugal fan of the present invention, the base-portion-side end of the impeller portion is circular or annular, and its central axis agrees with the central axis, a depression locally depressed in the direction separating from the impeller portion along the circumferential direction is formed in the base portion, and a space between the end and the base portion expands in the depression, and is substantially constant in other portions.

According to the present invention, formation of the depression enables efficient supply of air to the armature so as to suppress an increase in temperature of the armature.

Other features, elements, steps, advantages and characteristics of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a configuration of a centrifugal fan according to a first embodiment;

FIG. 2 is an exploded oblique view of the centrifugal fan;

FIG. 3 is a plan view showing a housing body and stator portion;

FIG. 4 is a view showing another example of a rotor portion;

FIG. 5 is a plan view showing another preferred example of the housing body;

FIG. 6 is a plan view showing another preferred example of the housing body;

FIG. 7 is a plan view showing a housing body of a centrifugal fan according to a second embodiment;

FIG. 8 is an oblique view showing the housing body of the centrifugal fan according to the second embodiment;

FIG. 9 is a sectional view showing a configuration of a centrifugal fan according to a third embodiment;

FIG. 10 is a sectional view showing the configuration of the centrifugal fan according to the third embodiment;

FIG. 11 is a view showing another example of the rotor portion;

FIG. 12 is a view showing another example of the rotor portion; and

FIG. 13 is an oblique view showing another preferred example of the housing body.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 is a view showing a configuration of a centrifugal fan 1 according to a first embodiment of the present invention. FIG. 1 shows a vertical sectional view obtained by cutting with a plain including a central axis J1. Further, FIG. 2 is an oblique view showing an exploded main configuration of the centrifugal fan 1. As shown in FIG. 1, the centrifugal fan 1 comprises: an impeller portion 2 which is rotated to generate the flow of air; and a motor 3 which is connected to the impeller portion 2 and rotates the impeller portion 2 around the central axis J1. The impeller portion 2 and the motor 3 are housed in a housing 10. As shown in FIGS. 1 and 2, the housing 10 is assembled by fitting a cover portion 12 having an inlet 121 to the housing body 11, and surrounds the impeller portion 2 to form a flow channel for the air (namely, the flow of the air that is generated by the rotation of the impeller portion 2 is adjusted for delivery). The centrifugal fan 1 is used, for example, as an electric fan for cooling an electric appliance or an electronic device.

FIG. 3 is a plan view of the housing body 11 and a later-described stator portion 31 in FIGS. 1 and 2. As shown in FIGS. 1 to 3, the housing body 11 as part of the housing 10 comprises: a base portion 112 to which the stator portion 31 is fitted; and a side wall portion 113 covering the periphery of the impeller portion 2, and has a configuration where the width of the flow channel between the side wall portion 113 and the impeller portion 2 gradually expands toward an outlet 13 (see FIG. 3). The side wall portion 113 comprises: a delivery wall 1131 on the outlet 13 side; and a curved wall 1132 located on the upstream side of the delivery wall 1131 (namely a portion less close to the outlet 13 along the inside-surface of the housing body 11).

In addition, in the centrifugal fan 1, while the delivery wall 1131 is flat and the curved wall 1132 is almost cylindrical with the central axis J1 as the center, the delivery wall 1131 may be curved to some extent. Also in this case, a (average) curvature radius of a substantially flat portion corresponding to the delivery wall 1131 is made larger than a (average) curvature radius of the curved wall 1132, and the portion where significant expansion of the width of the flow channel begins by the large change in curvature radius can be specified as a boundary of the delivery wall and the curved wall (corresponding to the position shown by numeral 1133 in FIG. 3). Moreover, as shown in FIGS. 2 and 3, a step 1134 is provided for reducing the space between the impeller portion 2 and the base portion 112 in the vicinity of the outlet 13 so as to suppress return of the flow of the generated air to the uppermost stream of the flow channel.

As shown in FIG. 1, the motor 3 is an outer rotor type motor, comprising a stator portion 31 as a fixed assembly and a rotor portion 32 as a rotational assembly. The rotor portion 32 is rotatably supported by a later-described bearing mechanism with the central axis J1 as the center with respect to the stator portion 31. In the following description, the rotor portion 32 side is the upper side and the stator portion 31 side is the lower side along the central axis J1 for the sake of convenience, but the central axis J1 is not necessarily required to agree with the gravity direction.

The stator portion 31 is fixed to the base portion 112 as the lower surface of the housing body 11, and holds each portion of the stator portion 31. A baring holding portion 311 in substantially cylindrical form with the central axis J1 as the center, which protrudes from the base portion 112 to the upper side (namely, the rotor portion 32 side), is fitted to a cylindrical portion formed at the midsection of the base portion 112. Inside the baring holding portion 311, ball bearings 312, 313 each as of the bearing mechanism are provided on the upper portion and the lower portion in the direction of the central axis J1, and a preload spring 314 is provided on the lower side of the ball bearing 313.

The stator portion 31 further comprises: an armature 315 fixed to the periphery of the baring holding portion 311 (namely, fixed to the base portion 112 on the periphery of the baring holding portion 311); and a circuit board 316, which is electrically connected to the armature 315 between the armature 315 and the base portion 112 and on which an electronic component for current control for the armature 315 is mounted.

The rotor portion 32 comprises: a cup portion 321 in substantially bottomed cylindrical form with the central axis J1 as the center, in which an opening 3211 is oriented downwardly (namely, the opening 3211 is opposed to the base portion 112); a substantially cylindrical field magnet 322′ fixed to the inside surface of the cup portion 321; and a shaft 323 protruding from the bottom of the cup portion 321 (namely, a substantially circular plate-like portion at the upper end of the cup portion 321) to the lower side.

The shaft 323 is pressed and fixed to the bottom of the cup portion 321. The shaft 323 is inserted into the baring holding portion 311 and rotatably supported by the ball bearings 312, 313. A latch member 3231 for latching the spring 314 is fitted to the vicinity of the lower end of the shaft 323, and since the spring 314 gives pre-load to the ball bearing 313, the shaft 323 and the ball bearings 312, 313 are held in appropriate positions. In the centrifugal fan 1, the shaft 323 and the ball bearings 312, 313 serve as a bearing mechanism, which rotatably supports the cup portion 321 with the central axis J1 as the center with respect to the base portion 112. A driving current supplied to the armature 315 through the circuit board 316 is controlled to generate a torque (namely, rotational force) with the central axis J1 as the center between the armature 315 and the field magnet 322 so that the shaft 323 and the impeller portion 2 fitted to the cup portion 321 rotate along with the cup portion 321 with the central axis J1 as the center.

As shown in FIGS. 1 and 2, the impeller portion 2 comprises: a plurality of blades 22 which are connected to the cup portion 321 and annularly arranged outside the cup portion 321 (apart from the cup portion 321) with the central axis J1 as the center; and an annular connection plate 23 which expands from the lower-side opening 3211 of the cup portion 321 toward the outside (namely in a direction separating from the central axis J1), and connects the plurality of blades 22 on the lower side (namely, the base portion 112 side). It is to be noted that the inside of the cup portion 321 is composed of a substantially bottomed cylindrical yoke member 3212 including a cylindrical yoke portion 321 a with the central axis J1 as the center. The yoke member 3212 is formed of a metal having magnetism, and the field magnet 322 is fitted to the inside surface of the yoke portion 321 a, and is opposed to the armature 315. The outside of the cup portion 321 is composed of a substantially bottomed cylindrical outside cup portion 3213 made of a resin, and shaped integrally with the whole of the impeller portion 2. The structure of the cup portion 321 is not limited to the structure of FIG. 1. For example, the yoke member 3212 may be cylindrically shaped, or as shown in FIG. 4, the outside cup portion 3213 may be omitted and the impeller portion 2 may be connected directly to the opening of the yoke member 3212.

Each of the plurality of blades 22 extends in substantially parallel with the central axis J1 from the upper surface of the connection plate 23 (namely, the bottom-side main surface of the cup portion 321), and the upper ends of the plurality of blades 22 are connected by an annular portion 24. In the centrifugal fan 1, the base portion 112 covers the opening 3211 side of the cup portion 321, and the inlet 121 opposed to the bottom of the cup portion 321 is formed in a cover portion 12. By rotation of the impeller portion 2 along with the cup portion 321, air is taken in from the inlet 121 and discharged in the direction separating from the central axis J1. The air flows along the flow channel between the impeller portion 2 and the side wall portion 113, to be led to the outlet 13.

As shown in FIGS. 1 to 3, the base portion 112 includes an annular protrusion 1121 protruding toward the connection plate 23 of the impeller portion 2. As shown in FIG. 1, a prescribed small space 14 is formed between the connection plate 23 and the base portion 112 by the annular protrusion 1121. The size of the space 14 is determined based upon the figuration accuracy and assembly accuracy of the impeller portion 2 and the base portion 112 so as to prevent the impeller portion 2 and the base portion 112 from coming into contact with each other. When the space between the connection plate 23 and the base portion 112 is large, stagnation of the air flow occurs to cause an increase in noise value in the centrifugal fan 1. In the centrifugal fan 1, the space between the connection plate 23 and the base portion 112 is made small in an attempt to suppress the noise value.

However, when the space 14 is made small for making the noise value small, a space formed by the cup portion 321 and the base portion 112 comes into close to a sealed state. Since the armature 315 and the circuit board 316 are located in this space, the temperatures of the armature 315 and the circuit board 316 rise. In the centrifugal fan 1, as shown in FIGS. 1 to 3, a depression 1122 is provided in the annular protrusion 1121 for partially expanding the space 14 (see FIG. 1) in the range from 30 to 180 degrees (preferably from 60 to 120 degrees, and further preferably from 80 to 100 degrees) with the central axis J1 as the center between the curved wall 1132 of the side wall portion 113 and the central axis J1. Moreover, from the viewpoint of efficiently supplying the armature 315 with the air, the depression 1122 is preferably provided from the position opposed to the boundary 1133 (see FIG. 3) of the delivery wall 1131 and the curved wall 1132 toward the upstream side.

In other words, in a sector-shaped region inside the curved wall 1132, the depression 1122 is preferably provided in the annular protrusion 1121 in a direction opposite to the rotational direction of the impeller portion 2 from the crossing position of a line connecting the boundary 1133 and the central axis J1 and the annular protrusion 1121 in the range from 30 to 180 degrees with the central axis J1 as the center. In addition, the boundary of the annular protrusion 1121 and the depression 1122 is arranged to be an inclined surface where the annular protrusion 1121 gradually becomes lower towards the depression 1122, and the air is smoothly led into the cup portion 321. In FIG. 3, a region of the bottom of the depression 1122 is provided with parallel oblique lines.

In the centrifugal fan 1, the depression 1122 reduces the degree of sealing of the space formed by the cup portion 321 and the base portion 112, and by the rotation of the impeller portion 2, the air is supplied to the stator portion 31 through the depression 1122.

As thus described, in the centrifugal fan 1, while provision of the annular protrusion 1121 protruding toward the connection plate 23 on the base portion 112 for making the space 14 small suppresses stagnation of the air flowing along the space 14 to make the noise value small, part of the annular protrusion 1121 is provided with the depression 1122 for expanding the space 14 and the air is thereby supplied to the space formed by the cup portion 321 and the base portion 112 so that the temperature rise of the armature 315 can be reduced.

In particular, with respect to the perpendicular direction to the central axis J1, the depression 1122 is opposed to the space between the armature 315 and the circuit board 316 to efficiently supply the air to both the armature 315 and the circuit board 316, thereby enabling suppression of the temperature rise of the armature 315 and the electronic component mounted on the circuit board 316. This can result in faster rotation of the impeller portion 2 of the centrifugal fan 1 along with improvement in derating of the electronic component, thereby allowing improvement in air volume characteristic. Further, with heat generation of the armature 315 suppressed, it is possible to suppress the temperature rise of the vicinity of the bearing mechanism so as to extend the life of the centrifugal fan 1.

Furthermore, in the centrifugal fan 1, the connection plate 23 continues to the opening 3211 of the cup portion 321 to cause blockages in the spaces between the plurality of blades 22 and the cup portion 321, thereby preventing generation of the air flow from the inside of the blades 22 to the armature 315. Hence it is particularly preferable to provide the depression 1122 on the base portion 112. In addition, while the space 14 can also be made small by making the impeller portion 2 high instead of providing the annular protrusion 1121, a current value of the motor 3 becomes high when the plurality of blades 22 become unnecessarily long. Therefore, when the consumption power is desired to be suppressed while maintaining the air volume characteristic, it is preferable to provide the annular protrusion 1121 and then provide the depression 1122 on the annular protrusion 1121.

In the above-mentioned structure, the more the range of the depression 1122 expands, the more the cooling efficiency of the armature 315 improves. However, since mere expansion of the depression 1122 might cause an increase in noise value, the depression 1122 is provided at the position and in the range harmonious for suppressing the noise value and the temperature rise of the armature 315. The position and the range where the depression 1122 is to be provided depend upon the figurations of the impeller portion 2 and the housing 10, a required air volume, static pressure characteristic, and the like, and the design of the depression 1122 is changed as appropriate according to application of the centrifugal fan 1.

FIGS. 5 and 6 are plan views each showing another preferred example of the housing body 11. It should be noted that, also in FIGS. 5 and 6, the bottom region of the depression 1122 is provided with parallel oblique lines as in FIG. 3. In the example shown in FIG. 5, the depression 1122 for expanding the space 14 (see FIG. 1) is provided in the annular protrusion 1121 from the position on the annular protrusion 1121 which is almost opposed to the boundary 1133 of the delivery wall 1131 of the side wall portion 113 and the curved wall 1132 toward the upstream side, and the range for the provision is 180 degrees with the central axis J1 as the center. Thereby, the air is supplied to the stator portion 31 (see FIG. 3) from the broader range.

Moreover, in the example shown in FIG. 6, the depression 1122 is provided in the annular protrusion 1121 from the upstream side of the position on the annular protrusion 1121 which is opposed to the boundary 1133 of the delivery wall 1131 of the side wall portion 113 and the curved wall 1132 toward the further upstream side, and the range for the provision is 90 degrees with the central axis J1 as the center. Since pressure is typically high on the upstream side, in the case of the example shown in FIG. 6, it is possible by provision of the depression 1122 on the upstream side to efficiently lead the air to the armature 315.

In the examples of FIGS. 3, 5 and 6, generally, the position of the depression 1122 is expressed as the position opposite to the outlet 13, and with the depression 1122 provided in such a manner, the air is efficiently supplied to the armature 315. The air supplied from the depression 1122 is then flown out from the space 14 on the downstream side toward the outlet 13.

Next, a centrifugal fan according to a second embodiment of the present invention is described. The centrifugal fan according to the second embodiment includes a housing body 11 a shown in the plan view of FIG. 7 in place of the housing body 11 shown in FIGS. 1 to 3, and other configurations are the same as those of the centrifugal fan 1 of FIG. 1. FIG. 8 is an oblique view of the housing body 11 a, and in FIGS. 7 and 8, the same constituents as those of the centrifugal fan 1 are provided with the same numerals as those of the centrifugal fan 1.

As shown in FIGS. 7 and 8, the base portion 112 of the housing body 11 a is provided with: a depression 1122 a for expanding the space 14 (see FIG. 1) between the connection plate 23 and the base portion 112 on the upstream side of the position on the annular protrusion 1121 which is opposed to the boundary 1133 of the delivery wall 1131 and the curved wall 1132; and another depression 1122 b for expanding the space 14 between the connection plate 23 and the base portion 112 at the position closer to the outlet 13 than the depression 1122 a. It should be noted that in FIG. 7, the regions of the depressions 1122 a, 1122 b are shown by parallel oblique lines, as in FIG. 3.

The depression 1122 b is provided so as to include the position on the annular protrusion 1121 which is opposed to the boundary 1133 of the delivery wall 1131 and the curved wall 1132. The air is taken in from the depression 1122 a on the upstream side where the pressure of the air becomes larger, and the supplied air is discharged from the depression 1122 b to the stator portion 31 (see FIG. 3). It is thereby possible to efficiently take in the air to the armature 315. In addition, the depression 1122 b may be provided on the downstream of the position opposed to the boundary 1133.

FIG. 9 is a vertical sectional view of a centrifugal fan 1 a according to a third embodiment of the present invention. The centrifugal fan 1 a is significantly different from the centrifugal fan 1 in that a plurality of blades 22 a radiate from the outside surface of the cup portion 321. Namely, while the centrifugal fan 1 of FIG. 1 is a so-called sirocco fan, the centrifugal fan 1 a of FIG. 9 is a so-called turbo fan. Further, in the centrifugal fan 1 a, the annular protrusion 1121 of the base portion 112 is provided so as to protrude toward (the edge of) the opening 3211 of the cup portion 321, and the annular protrusion 1121 is provided with the depression 1122 for partially expanding the space between the opening 3211 of the cup portion 321 and the base portion 112. Other structures are the same as those of the centrifugal fan 1 of FIG. 1, and the same numerals are provided.

In the case of the centrifugal fan 1 a shown in FIG. 9, as in the case of FIG. 1, the air is supplied to the armature 315 inside the cup portion 321 through the depression 1122, thereby to allow suppression of the temperature rise of the armature 315.

FIG. 10 is a vertical sectional view of a centrifugal fan 1 b according to a fourth embodiment of the present invention. In the sirocco-fan type centrifugal fan shown in FIG. 1 and the housing body shown in FIG. 7, the depressions 1122, 1122 a, 1122 b for partially expanding the space 14 are formed at the position opposed to the connection plate 23. Meanwhile in the centrifugal fan 1 b shown in FIG. 10, the depressions and the space 14 are provided as opposed to the (edge of) the opening 3211 of the cup portion 321 as in the case of the turbo-fan type centrifugal fan shown in FIG. 9. Other respects are the same as those of the first embodiment.

As shown in FIG. 10, also in the case of providing the depression 1122 of the annular protrusion 1121 not on the lower side of the connection plate 23 but below the opening 3211 of the cup portion 321, it is possible to efficiently supply the air to the armature 315 and the circuit board 316 and suppress the temperature rise of the armature 315 and the circuit board 316, as in the first embodiment.

It is to be noted that, even in the case of providing the depression 1122 between the opening 3211 of the cup portion 321 and the base portion 112 as shown in FIGS. 9 and 10, the range of the depression 1122 may be changed as in FIGS. 5 and 6, and a plurality of depressions may be provided as shown in FIG. 7.

In addition, even in the case of providing the connection plate 23 on the side surface of the cup portion 321 as in the case of the rotor portion 2 shown in FIGS. 11 and 12, the structure where the depression 1122 is provided between the opening 3211 and the base portion 112 as in FIG. 10 can be utilized.

While the embodiments of the present invention were described above, the present invention is not limited to the above embodiments, and a variety of alterations can be made.

For example, in the centrifugal fan 1 shown in FIG. 1, the annular protrusion 1121 for making the space between the connection plate 23 and the base portion 112 small is formed on the base portion 112, and the part of the annular protrusion 1121 is provided with the depression 1122. However, as shown in FIG. 13, a depression 1122 c may be formed in the base portion 112 which has been brought into a flat state, and air for cooling may be supplied from the depression 1122 c into the cup portion 321. It is to be noted that the depression 1122 c with a depth not larger than the thickness of the base portion 112 may be provided at the position of the depression 1122 c, and the rear surface of the base portion 112 may be made flat, or the base portion 112 may be protruded downward in the depression 1122 c.

Moreover, the range where the space 14 is expanded by the depression provided in the base portion 112 is not required to be limited to the foregoing angles and numbers, and for example, not less than three depressions may be provided in the annular protrusion 1121.

While the whole of the armature 315 is housed into the cup portion 321 in the above embodiments, in the case of arranging the armature 315 in the depression formed around the central axis J1 of the base portion 112, the armature 315 may be partially located inside the cup portion 321. Further, a sleeve bearing may be used other than the ball bearing in the bearing mechanism.

While the present invention has been described with respect to preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the present invention which fall within the true spirit and scope of the invention. 

1. A centrifugal fan, comprising: a substantially bottomed cylindrical cup portion, including a cylindrical yoke portion with a prescribed central axis as the center; a field magnet, arranged inside the cup portion and fixed to the cup portion; an impeller portion, connected to the cup portion, having a plurality of blades arranged around the central axis outside the cup portion, and rotated along with the cup portion to take in air from the bottom side of the cup portion and discharge the air in a direction separating from the central axis; a housing, having a side wall portions which covers the periphery of the impeller portion and has a pair of ends in a circumferential direction, a base portion which covers the opening side of the cup portion and a cover portion in which an inlet opposed to the bottom of the cup portion is formed, the housing being where a width of a flow channel between the side wall portion and the impeller portion gradually expands toward an outlet as an opening between the pair of ends; a bearing mechanism, which rotatably supports the cup portion with the central axis as the center with respect to the base portion; and an armature, which is fixed to the base portion and at least partially located inside the cup portion, and generates a torque with the central axis as the center between the located position and the field magnet, wherein the base-portion-side end of the impeller portion is circular or annular, and its central axis agrees with the central axis, a depression locally depressed in the direction separating from the impeller portion along the circumferential direction is formed in the base portion, and a space between the end and the base portion expands in the depression, and is substantially constant in other portions.
 2. The centrifugal fan according to claim 1, wherein the impeller portion includes a connection plate for connecting the plurality of blades at the base-portion-side portions of blades, and the connection plate constitutes the base-portion-side end of the impeller portion.
 3. The centrifugal fan according to claim 2, wherein the connection plate is an annular plate having a width in a radial direction, and the inner edge of the annular plate in the radial direction is located on the peripheral surface or the inside of the cup portion.
 4. The centrifugal fan according to claim 1, wherein the plurality of blades radiate from the outside surface of the cup portion.
 5. The centrifugal fan according to claim 1, wherein the depression spreads in the circumferential direction in the range between not smaller than 30 degrees and not larger than 180 degrees with the central axis as the center, and the end of the depression on the downstream side in a rotational direction of the impeller portion is located on the upstream side of the downstream-side end out of the pair of ends of the side wall.
 6. The centrifugal fan according to claim 5, wherein the base portion includes another depression for expanding the space between the base-portion-side end of the impeller portion and the base portion.
 7. The centrifugal fan according to claim 1, wherein the base portion includes an arc-shaped protrusion protruding toward the base-portion-side end of the impeller portion, and between the ends of the arc-shaped protrusion, a space formed with a bottom of the impeller portion expands to give the depression.
 8. The centrifugal fan according to claim 1, further comprising, between the armature and the base portion, a circuit board on which an electronic component for controlling a current to be supplied to the armature is mounted, wherein the position of the electronic component in an axial direction substantially agrees with the position of the depression in the axial direction.
 9. The centrifugal fan according to claim 5, wherein the base portion includes an arc-shaped protrusion protruding toward the base-portion-side end of the impeller portion, and between the ends of the arc-shaped protrusion, a space formed with the bottom of the impeller portion expands to give the depression. 